This invention relates to vacuum fluorescent displays (VFDs) and more particularly to improvements allowing much higher brightness level to be obtained than in such displays known in the art.
In recent years, vacuum fluorescent display (VFD) devices, utilizing phosphor display elements to form a viewed alphanumeric or graphic image, have come into wide use as displays in electronic and electrical appliances, such as audio receivers, calculators, measuring instruments and xerographic copiers. They are now also of growing interest for automobiles as a replacement for the cluster of electro-mechanical dashboard meters. VFD displays are basically three element vacuum tubes whose envelope is at least in part transparent. The envelope encloses, in spaced relation, a directly heated filamentary cathode, a set of metallic control grids and phosphor coated anode strips or segments. Upon application of a prescribed voltage across the ends of the oxide coated cathode filament or set of filaments, its temperature is raised to about 650.degree. C. causing it to emit electrons and produce an electron flow. The control grids are located between the cathode and the anodes and through the application of a positive voltage with respect to the cathode to a selected grid electrode, the electrons are drawn from the cathode through this grid to the neighborhood of the anodes adjacent to the grid. If the grid is held at a negative potential, electron flow to the corresponding anodes is cut off.
Those electrons that pass through a positive grid and approach the surface of a particular anode will be repelled by the anode and prevented from landing if the anode is at the same potential or a negative potential with respect to the cathode. However, if a particular anode and its overlying selected grid are simultaneously made positive (anode addressing), electrons that pass through the grid will land on the anode and, because of their kinetic energy, will excite the phosphor coating of this anode, generating luminescence.
For applications requiring a large number of display elements, VFDs have been developed in which the grids and display anodes are arranged in the form of two sets of mutually orthogonal strips. In operation, successive grids corresponding to the rows of the display are pulsed positive while appropriate input signals in the form of positive pulses are simultaneously applied to the anode strips. As the number of rows of display elements is increased, however, the fraction of the time that a particular display element is on, i.e., its duty cycle, is reduced so that higher and higher voltage is required on the display anodes to produce a given average brightness. For example, a vacuum fluorescent display having 256 rows of disply elements required pulse voltages applied to the display anodes to be raised to about 150 volts in order to produce an average brightness of about 25 foot lamberts. For displays with a higher resolution, i.e., a significantly greater number of rows of elements, a corresponding increase in the magnitude of the anode pulse voltages is required to maintain the brightness. Where an even higher brightness is needed, a further increase in the pulse voltages is required. Aside from problems of panel design, operation at such high voltages, e.g. in excess of 200 volts, results in problems of power dissipation and requires special, relatively costly, integrated circuit devices capable of handling these high voltage and power levels. Prior art VFD devices are thus limited in brightness and resolution because of these considerations.
It is a principal object of this invention to achieve a high degree of brightness in vacuum fluorescent displays without being limited by the sequential addressing of successive rows of display elements. Another object of this invention is to provide a flicker-free display which can be addressed at an arbitrarily slow rate.